1. Field of the Invention
This invention relates to an alumina-based wiring substrate having a high dielectric layer in its inside and a package for semiconductor device, and more specifically, to a wiring substrate and a package having stable electric properties, excellent sealing performacne, and mechanical strength.
2. Description of the Prior Art
Heretofore, for Example, in a package for semiconductor device, semiconductor ICs (integrated circuits) are liable to make an erroneous action by foreign noises or unwanted irradiation. By building-in a ceramic capacitor having a capacity of about 30 to 100 uF between a power supply side and a ground side, the noises are absorbed to prevent the erroneous action. Since the connection of this capacitor was heretofore separately performed outside of the package, the density of wiring or the operation speed of IC cannot be increased.
From this viewpoint, a package containing a capacitor built in was recently developed (for example, see Japanese Laid-Open Patent Publication No. 160461/87). On the other hand, since alumina used as a substrate material heretofore is an insulator and a dielectric, it may be used as a dielectric, and a capacitor may be built in a package.
However, such an alumina has a low permittivity of about 0.5 to 10. Accordingly, when a dielectric is formed from alumina itself, it was necessary to form a plurality of dielectric layers interposed with electrodes and to increase the area of the dielectric. For this reason, the package or substrate becomes large in size, and the cost of the product increases.
Furthermore, for example disclosed in Japanese Laid-Open Patent Publication 87091/1991, which corresponds to U.S. Pat. No. 5,099,388, issued Mar. 24, 1992, a multi-layer alumina-based wiring substrate is known in which a high dielectric layer containing a high permittivity-imparting agent composed of W or Mo in the alumina is interposed between insulation layer containing alumina as a main component and a pair of electrode layers composed of a paste of a high-melting metal such as W or Mo as a main component coated or printed are formed on both sides of the high dielectric layer.
However, the known high dielectric layer-containing wiring substrates or packages for semiconductor device are still unstable in electric properties, and an insulation resistance between electrodes is decreased. In extreme cases, there is a fear that the electrodes are likely to be shorted. Package has insufficient sealability (air-tightness). Moisture may permeate to change the surface resistance of the electrode layers. There is also a defect that a sufficient adhesion strength cannot be obtained between the electrode layer and the high dielectric layer or the insulation layer.